Upon pathogen attack, plants respond by activating their defense mechanisms. The activated tran-
scription machinery of plants is exploited by pathogens to establish colonization on their host. It is ap-
parent that plant viral and bacterial pathogens have coevolved with higher plant defense systems and
recruitedas-1–typeciselements for their gene promoters in order to counteract plant defense systems.
This indicates that as-1–type elements are critical in regulating defense-related genes under biotic stress
conditions.
III. THE as-1–TYPE ELEMENTS ARE ONE CLASS OF STRESS-
RESPONSIVE ELEMENTS WIDELY USED BY DEFENSE-RELATED
GENES IN HIGHER PLANTSAlthoughas-1–typeciselements were initially identified and characterized in plant pathogens Agrobac-
terium tumefaciensand cauliflower mosaic virus, it is conceivable and expected that this type of cisele-
ment is authentic to the promoters of plant genes that play important roles in defense against both biotic
and abiotic stresses. Indeed, more and more plant genes have been found to possess as-1–typecisele-
ments in their promoters.
A. The as-1–Type Elements Are Found in the Promoters of
Pathogenesis-Related Genes of Higher PlantsPathogenesis-related (PR) genes are coordinately induced in higher plants during the onset of systemic ac-
quired resistance (SAR). SAR is a well-characterized plant defense mechanism [13] that is triggered upon
pathogen infection or by exposure to salicylic acid (SA). The promoter of the PR-1gene, one of the genes
encoding PR proteins, has been subjected to extensive functional analyses [10,14]. Strompen et al. [14]
identified a region of 139 bp (from 691 to 553) in the PR-1agene promoter responsive to SA and re-
vealed an as-1–like element with two TGACG motifs within this region. TGA1a, a bZIP transcription fac-
tor known to bind the as-1element specifically, bound this element in vitro with specificity and affinity
similar to those of as-1in gel shift assays. Mutations in this element in the context of the PR-1apromoter
caused significant reduction of reporter gene activity. Lebel et al. [10] delineated the SA-responsive re-
gion from 640 to 610 within the PR-1promoter using linker-scanning mutagenesis. A bZIP transcrip-
tion factor binding site was identified in the 640 region and a consensus binding site for the transcription
factor nuclear factor B (NF-B) was identified in the 610 region. In vivo footprinting results showed
tight correlation with the functionality of these promoter regions. It has been demonstrated that TGA2, an-
other bZIP transcription factor in Arabidopsis(see later), binds to the 640 region of the PR-1promoter
[15]. A similar element was identified in another PR gene (PRB-1b) promoter [16]. These studies have
firmly established the functional importance of the as-1–type elements in activating defense genes in re-
528 XIANG
Figure 1 Theas-1cognate promoter elements in plant pathogen promoters and plant defense-related gene
promoters. The sequences are compiled from the references cited. The half site core sequences are underlined.
The two repeats can be in any orientation. The AP-1 binding site of mouse GST Ya gene is also shown for
comparison.